This invention relates to a shielding device for use in radio telephones, preferably for the shielding of electronic components on a circuit board.
Radio telephones are required by law to adhere to ElectroMagnetic Compatibility (EMC) limits as laid down in Type Approval Specifications for Mobile Phones. EMC is defined as the ability of a device to function properly in its intended electromagnetic environment and not to be a source of electromagnetic pollution to that environment.
To enable electronic apparatus to be electromagnetically compatible, electromagnetic (EM) shielding is often placed within the apparatus to reduce the radiation emissions from radiating sources and to reduce the levels of radiation reaching sensitive components.
In general, EM shields take the form of metal or metallised plastic covers designed to be placed over electronic components on a circuit board. To prevent the passage of radiation towards or away from the electronic components the shields are electrically grounded, typically using the grounding planes present on the circuit board. The most common way of attaching the shields to the circuit board and particularly to the grounding planes is by soldering. This provides a semi-permanent fixing that may only be removed by de-soldering.
Ordinarily, during the production of radio telephones the electronic components are firstly mounted onto the circuit board. EM shields are then placed over the components and secured using soldering techniques that may also provide electrical coupling to ground. The circuit board supporting the electronic components and the shield can then be tested. A percentage of circuit boards inevitably fail the tests and inspection and repair of the electronic components is usually required. Similarly, units which pass the tests and subsequently become faulty for any reason may also require inspection and repair.
Current soldered shield designs have a disadvantage in that the inspection and repair of soldered electronic components is blocked by the presence of the EM shield casings. To inspect and repair the electronic components the shields must be de-soldered, the inspection and repair carried out and then the shields must be re-soldered. De-soldering and re-soldering is laborious and creates a risk of damage to electronic components within the region of the shield.
Other known shielding devices such as U.S. Pat. No. 4,890,199 have the EM shield casing attached to the circuit board using one or more gaskets which allow removal of the shield casing during repair. These gaskets are, however, generally complex in structure and have to be mounted individually onto the circuit board during construction of the shield. Consequently manufacturing cost and time are increased using this method.